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Q. 2

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Physics for Scientists and Engineers: A Strategic Approach with Modern Physics
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Short Answer

FIGURE Q24.2 shows cross sections of three-dimensional closed surfaces. They have a flat top and bottom surface above and below the plane of the page. However, the electric field is everywhere parallel to the page, so there is no flux through the top or bottom surface. The electric field is uniform over each face of the surface. For each, does the surface enclose a net positive charge, a net negative charge, or no net charge? Explain.

(a) The given figure has no net charge.

(b) The given figure has positive charges.

(c) The given figure has negative charges.

See the step by step solution

Step by Step Solution

Step 1 : Given information and formula used  

Given :

The given figures have a flat top and bottom surface above and below the plane of the page.

The electric field is everywhere parallel to the page; there is no flux through the top or bottom surface.

The electric field is uniform over each face of the surface.

Theory used :

Electric field is a vector quantity whose direction is defined as the direction that a positive test charge would be pushed when placed in the field. The electric field direction about a positive source charge is always directed away from the positive source.

Step 2 : Determining if the given surface enclose a net positive charge, a net negative charge, or no net charge 

(a) The amount of electric field that inwards the surface is the same as the amount of electric field that outwards the surface, as indicated in the diagram below.

So, there is no net electric flux. As a result, there is no net charge on the surface.

Step 3 : Determining if the given surface enclose a net positive charge, a net negative charge, or no net charge 

(b) The electric fields in the figure below point outward from the surface. The positive charges are directed outward by the electric fields, therefore the closed surface possesses positive charges.

Step 3 : Determining if the given surface enclose a net positive charge, a net negative charge, or no net charge 

(c) The electric fields in the illustration below point inward on the surface. Negative charges are attracted by the electric fields, hence the closed surface possesses negative charges.

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